1. Field of the Invention
The present invention relates to methods of making lamp bulbs of discharge lamps. The invention also relates to the lamp bulbs made by the methods. The invention in addition relates to the discharge lamps, in particular the miniaturized discharge lamps, made from the lamp bulbs.
2. Description of the Related Art
A lamp bulb is to be understood as meaning the discharge vessel in which the light is generated.
Miniaturized discharge lamps, which are known as backlights, are used for background illumination of, for example, displays, such as displays of personal computers, laptops, pocket calculators, vehicle navigation systems, flat screens and mobile telephones. Typical external diameters for miniaturized discharge lamps of this type are between 2 and 5 mm. Typical internal diameters are between 1 and 4.8 mm.
Because of the structure of standard discharge lamps, it must generally be possible for the glasses used to be fused to a metal or metal alloy which is suitable as an electrode and/or electrode feed. For this purpose, the glasses should have thermal expansion characteristics, which are matched to the thermal expansion characteristics of the metal or metal alloy and a transformation temperature, which is matched to the fusing temperature.
The glasses which are customarily used for discharge lamps, including in particular for backlights, therefore, have transformation temperatures Tg which are matched to a fused seal with alloys, such as KOVAR®, i.e. relatively low transformation temperatures of Tg<550° C.
In a special type of discharge lamp, known as the EEFL, which stands for external electrode fluorescent lamp, i.e. a discharge lamp without an internal electrode, which is also available in miniaturized form, the requirements which have been described with regard to thermal expansion and transformation temperature are not the primary considerations.
A significant property for glasses for discharge lamps of any type is the transmission curve of the glasses. In the visible region (VIS), a high light transmission is required in order to obtain high illumination efficiency from the discharge lamps. In the UV region, no transmission or a low transmission is desirable, in order to pass only the minimum possible amount of harmful UV radiation. Especially for backlights, a high UV blocking at ≦260 nm is desirable, so that irradiated plastic parts, for example in laptops, do not become yellow and brittle.
The requirement for transmission in the visible wavelength region between 400 nm and 800 nm is a transmission τ>90% with a specimen thickness of 0.2 mm. The requirement for transmission in the UV region <260 nm is τ<1% with a specimen thickness of 0.2 mm.
Resistance to solarization is a further important property for backlights. This is required so that the service life of the lamps is long, i.e. a light efficiency which remains as constant as possible.
The term “solarization-resistant” is to be understood as meaning in this context glasses whose transmission at λ=300 nm (specimen thickness 0.2 mm) drops by at most 10% after 15 hours of HOK-4 irradiation, i.e. irradiation with a high-pressure mercury lamp with a main emission at 365 nm and an irradiation strength of 850 μW/cm2 at 200 to 280 nm at a distance of 1 m.
The desired properties of transmission or blocking and solarization resistance can be achieved with the aid of dopants, for example by adding TiO2, Fe2O3 and/or CeO2.
U.S. Pat No. 4,047,067 discloses discharge lamps in which the lamp bulb consists of silica glass, which is coated with a layer of aluminosilicate glass. The coating of aluminosilicate glass is made by fusing aluminum oxide to the silica glass surface at high temperatures.
U.S. Pat. No. 4,751,148 discloses luminescent aluminoborate and/or aluminosilicate glasses, which are activated with rare earths. These glasses are used as luminescent layers on luminescent screens, for example for cathode ray tubes.
It is known to use borosilicate glasses for lamp bulbs for discharge lamps.
A drawback of the known borosilicate glasses used for discharge lamps is their relatively low thermal stability. This drawback restricts, for example, the upper limit for the firing temperature for the phosphor. The phosphors required, generally inorganic crystals such as silicates, tungstates, phosphates and aluminates of the rare earths, are applied to the glass as a suspension in high-molecular-weight organic binders, for example binders based on butyl rubber with collodion wool, in which case the organic binder would have to be completely evaporated out before the lamp is used in order for it not to have any adverse effect on the gas discharge. With conventional discharge lamp glasses, this is only achieved incompletely or is only achieved after a very long time and with yield losses on account of deformation of the glass.